The molten metal continuous casting method has been adopted over the whole world since 1960s. This method has various advantages compared with the general ingot making method, and therefore, it is utilized for a considerable part of the manufactured steel.
The quality of a continuously cast metal is classified into a surface quality and an internal quality, and these qualities are closely related to the flow of molten metal within the mould.
FIGS. 1a and 1b illustrate a mould used in the general continuous casting method. Referring to these drawings, a molten metal is supplied into a mould 10 through a submerged nozzle 11 which has two discharge holes 11a. The molten metal which is discharged from the two discharge holes forms jet flows toward a narrow face 13, and the jet flow collides with the narrow face 13 to be divided into an ascending flow U and a descending flow D. That is, the jet flow is divided into four recirculating streams U1, U2, D1 and D2. In FIG. 1b, reference code S indicates a turning point of the recirculating streams.
The molten metal which is introduced into the mold contains non-metallic inclusions (also called "inclusions" below) such as Al.sub.2 O.sub.3, MnO, SiO.sub.2 and the like which have been formed in the pre-treating stage or have come from the refractory materials. The molten metal further includes inert gas bubbles (also called "gas bubbles" below) which have been injected into the submerged nozzle 11, for preventing the clogging of the nozzle 11. The gas bubbles have sizes of several scores of microns to several millimeters. The inclusions and gas bubbles which are contained in the upper recirculating streams have a density lower than that of the molten metal. Therefore, they are subjected to a floating force in a direction opposite from gravity, and therefore, they move in the combined vector direction of the molten metal flow and the floating force. Then they gradually move toward the meniscus of the molten metal, to be captured by the mold flux 14.
However, the inclusions and gas bubbles which are contained in the lower recirculating streams D pass through the jet flow region near the nozzle discharge holes 11a before moving toward the upper recirculating streams U. The velocity of the jet flow is faster than the ascending velocity due to the floating force, and therefore, the inclusions and the gas bubbles rarely pass through the jet flow. Accordingly, the inclusions and the gas bubbles which are contained in the lower recirculating streams cannot reach the meniscus of the molten metal, but continuously circulate along with the lower recirculating streams. Therefore, they are likely to remain within the cast metal. Particularly, in the case of the continuous curved caster, the particles contained in the lower recirculating streams spirally move due to the influence of the floating force to be ultimately adhered on the solidified layer, i.e., on the upper layer of the cast piece, thereby forming an inclusion/gas bubble accumulated region in the upper layer of the cast piece.
When the cast piece is subjected to a rolling, the residual inclusions and gas bubbles are exposed to the surface, thus causing surface defects. Or they remain within the cast piece, and when an annealing is carried out, the gas bubbles expand to cause internal defects.
In order to solve this problem and to improve the quality of the cast piece, conventionally the discharge angle .THETA. of the submerged nozzle is properly adjusted, so as to improve the quality of the cast piece. The discharge angle .THETA. of the submerged nozzle gives a great influence to the flow of the molten metal.
If the discharge angle .THETA. is increased, the amount of the descending flow increases, while that of the ascending flow decreases. As a result, the velocity of the molten metal on the meniscus of the melt is slowed, so that a stable surface of the melt is maintained. Therefore, the workability is improved, and the initial solidification is stably carried out, thereby upgrading the surface quality of the cast piece. However, if the discharge angle .THETA. is increased, large amounts of inclusions and gas bubbles are buried deeply into the cast piece, because they lose the opportunity of floating to the meniscus of the melt. Thus the internal quality of the cast piece is aggravated.
On the other hand, if the discharge angle .THETA. is decreased, the amount of the descending flow decreases, and therefore, the defects due to the inclusions and the gas bubbles may decrease. However, if the discharge angle is decreased, the amount of the ascending flow increases, and the velocity of the molten metal at the meniscus of the melt steeply increases. Therefore, the surface quality of the cast piece is decreased due to the entrainment of the mould flux at the melt surface, and due to the formation of vortex. These problems become much more serious as the casting speed becomes faster.
Thus, if only the submerged nozzle is employed, a limit in controlling the flow of the molten is confronted. Therefore, as shown in FIG. 2a, an electromagnetic brake ruler (EMBR) 20 is installed immediately below the discharge hole 11a of the submerged nozzle. Thus the Lorentz force based on a magnetic field and a flow is utilized to decrease the flow velocity. (This is proposed in Swedish Patent SE 8,003,695, and U.S. Pat. No. 4,495,984.)
The method of FIG. 2a has been put to the practical use, but it is not used at present because flow distortions occur in the direction of evading the flow resistance of the magnetic field, rather than decreasing the flow velocity by the magnetic field.
In order to overcome this problem, the magnetic field is horizontally distributed over the entire width of the mould as shown in FIGS. 2b and 2c. (Swedish Patent SE 9,100,184, U.S. Pat. No. 5,404,933, and Japanese Patent Application Laid-open No. Hei-2-284750). However, the distortion phenomenon has been observed in these methods all the same.
When a dc magnetic field is not applied, the molten metal which has been discharged from the discharge holes 11a of the submerged nozzle 11 forms flow fields as shown in FIG. 3a. However, if the magnetic field is applied over the entire width of the mould, the flow steams are formed as shown in FIG. 3b. That is, compared with the case where there is magnetic field, the jet flow is markedly spread in the thickness direction of the mould. Therefore, the average velocity of the jet flow directed toward the mould narrow face is slowed.
As the velocity of the jet flow is slowed, the inclusions and the gas bubbles of several scores to several hundreds of microns have a long way to travel from the descending flow region to the ascending flow region, compared with the case where a magnetic field is not applied.
Meanwhile, most of the inert gas which has been injected through the nozzle into the molten metal has of several millimeters, and floats from between the narrow faces to the meniscus of the melt (the floating distance depends on the molten metal injection speed and on the amount of the injected gas, and this distance corresponds from near the discharge hole to the narrow face in the case where the minimum gas amount is injected, while it corresponds from immediately above the discharge hole to the narrow face in the case where the maximum gas amount is injected). If the velocity of the main flow is light, the direction of the main flow is not greatly affected by the floating of the inert gas bubbles. However, if the average velocity of the main flow is decreased by applying a magnetic field, the direction of the main flow is greatly influenced by the floating force of the inert gas. The main flow is raised toward the surface of the melt by the floating force of the inert gas and by the flow resistance of the magnetic field which is established immediately below the submerged nozzle. When the influence by the floating force of the inert gas decreases, the flow is lowered in the casting direction to draw an S curve as shown in FIG. 3b (this is called "non-solidified rising molten metal flow adjacent to the submerged nozzle"). Thus the flow collides with the mould narrow race with a large angle.
When the jet flow is cleaved by colliding with the narrow face of the mould, the flow amounts of the cleaved flows are decided by the colliding angle. For example, if a perpendicular collision occurs, the upper and lower cleaved flows are same in their flow amounts. However, if the colliding angle is lowered, the amount of the lower flow is increased. Under this condition, the ratio of the amount of the lower flow to that of the upper flow is decided by the casting speed, the nozzle discharge angle, the injected amount of the inert gas, and the magnetic field strength. However, at the general working conditions, the ratio is about 6:4, if a magnetic Field is not applied. If a magnetic field is applied over the entire width, the ratio becomes 8:2. Therefore, if a magnetic field is applied like in the conventional method, the amount of the lower flow increases, while the amount of the upper flow decreases. Accordingly, the velocity of the molten metal decreases immediately below the melt meniscus, and the height difference of the melt meniscus also decreases. Thus the melt face is stabilized, so as to improve the surface quality.
However, due to the increase in the amount of the lower flow, large amounts of inclusions and gas bubbles are contained in the recirculating flow. Therefore, if a magnetic field is applied over the entire width, the increase of the floating opportunity owing to the decrease of the average velocity is offset. Therefore, the improvement of the internal quality cannot be expected due to the fact that the inclusions and the fine inert gas bubbles are not removed.